On most modern wind turbines, the pitch angle of the blades relative to the rotor plane can be controlled continuously. Hereby the inflow angle to the blade can be varied which is used e.g. for rotor speed, power and load control. However, due to the large size and weight of the present-day blades, the response time on the pitch angle is increasing. Instead blades can be designed where it is possible to change the shape of the rear part section of the blade locally, whereby it is possible to react faster and alleviate more of the fluctuating loads and reduce the aerodynamic noise. Such a blade design is disclosed in WO 2004/088130 in which the changes in the shape deformable blade sections are obtained by use of actuators.
Numerical and experimental studies disclosed in (Buhl, T. and Gaunaa, M. and Bak, C., “Potential Load Reduction Using Airfoils With Variable Trailing Edge Geometry”, Journal of Solar Energy Engineering, 2005, vol. 127(4), pp. 503-516 and Troldborg, N. “Computational study of the Risø-B1-18 airfoil with a hinged flap providing variable trailing edge geometry”, Wind Engineering, vol. 29(2), 2005, pp. 89-113) have been performed to give indications on the effect of deformable airfoils.
Deformable airfoils are known from U.S. Pat. No. 6,015,115 related to aircrafts. The deformable airfoils can be used to modify the shape of the aircraft wing before or during flight for obtaining desired flight characteristics. It may e.g. be advantageous to have a larger wing thickness for low speed flight than for high speed flight.
Other designs of airfoils comprising deformable elements are shown in WO 02/051730 and WO 2007/045940.